Zero-day vulnerability has found in the Firefox browser, a JavaScript exploit — which consists of one CSS and one HTML file — has been discovered in the Wild and has been used to target Tor browser users now. Firefox security team is aware of this and has readied a patch Firefox 50.0.2, this will be released soon and will also be available to Firefox 50 users via update mechanism. FYI, yesterday released Firefox 50.0.1 update has addressed a different security issue, not this.